Summary:
Custom widget was reseving its options, even though they haven't been changed. This
resulted with an ACL error and a popup message in readonly mode.
Test Plan:
Existing and manual.
To recreate:
1. Create a page with a calendar widget (don't change anything)
2. View this page as a Viewer (using ACL dropdown in tools)
3. The error should be shown.
Reviewers: georgegevoian
Reviewed By: georgegevoian
Subscribers: georgegevoian
Differential Revision: https://phab.getgrist.com/D4060
